Geyser action is characterised by the eruption of a mixture of steam, water and, in some cases, mud to substantial heights above surface level. The role of vaporisation in the ejection process is investigated with the aid of a mathematical model which treats geyser eruptions as ioslated, steady state, one-dimensional phenomena; they are treated from an Eulerian point of view. It is found that large scale boiling of water occurs close to the surface, even though the instabilities that initiate the cycle may occur much lower down. Somewhat different models characterise superficially similar geyser eruptions from narrow confined tubes and from pools such as crater lakes. In the latter case the model predicts a root zone for steam flash at a depth of a few meters below the water surface, compared to a depth of tens of metres below the exit plane when eruption is from a confining tube, for normal eruptions ten to one hundred metres high. Relationships are obtained between geyser height, water content, depth of vaporistation, and such parameters as temperature and pressure at the onset of vaporisation. These relationships are used to provide a simple model of an eruption of Old Faithful, which reflects it main characteristics. They are also used to discuss geysering in the crater lake of Po¿s volcano in Costa Rica. A triggering mechanism which depends on progressive compression of a volume of steam is considered briefly. The systems ultimately becomes unstable and results in the ejection of a volume of fluid. ¿American Geophysical Union 1991